1. Technical Field
The present disclosure refers to satellite positioning systems, and, to acquisition techniques of CDMA-type satellite signals.
2. Description of the Related Art
The satellite signals used in the GNSS (Global Navigation Satellite System) field are CDMA (Code Division Multiple Access) type signals.
The CDMA-modulated signals have a radio frequency carrier amplitude-modulated by a suitable binary sequence known as spreading sequence or pseudo-random code (PRNC: Pseudo Random Noise Code) having a plurality of pulses called chips. The pseudo-random codes have excellent orthogonality properties which enable to distinguish a satellite from another by the same.
The reception of satellite signals from the receiving apparatus comprises the following standard steps, sequentially executed: frequency conversion and digitization, acquisition, tracking, decoding, and positioning.
A radio frequency stage operates on the analog signals received at the satellites and converts them to an intermediate frequency, and an analog/digital converter converts the intermediate frequency fint signals to corresponding digital signals.
The intermediate frequency fint converted signal shows frequency shifts due to shifts of the local oscillator used for the conversion, and due to Doppler effects caused by the satellites and receiving apparatus motions.
During the acquisition of CDMA signals of satellites, there are two main operative conditions: a “Cold Start” state and a “Hot/Warm Start” state.
In the “Cold Start” state, the Doppler shift caused by the satellite and receiving apparatus motions is considered that is completely unknown.
In the “Hot/Warm Start” state, the largest Doppler uncertainty is considered null because in such state it is possible to predict both which are the satellites in view and their speeds by assuming known the position (corresponding to the one available at the preceding turning off) and the time (generated by the inner backup clock).
Further, there is a “reacquisition” state which occurs when the receiving apparatus looses the synchronization with the satellite signal (for example as it occurs when the somebody passes through a tunnel) but it is assumed that it is possible to continuously estimate the code phase of the signal from the satellite with a precision of few chips from the last tracked code, from the elapsed time and from the tracking frequency available before the obscuration.
Document EP-A-1321774 describes a reacquisition method of a satellite signal, and shows an acquisition block performing the generation of a plurality of CDMA reference signals, and calculates the correlation of these signals with a received intermediate frequency signal. The CDMA reference signals have different code phases. Each correlation calculation is parallelly performed in two different processing branches. A first processing branch comprises calculation blocks (accumulation, sum, coherent value formation, filtering, and comparison blocks) operating in time intervals having a duration less than (fast reacquisition) the time intervals during which the blocks (accumulation, sum, incoherent value formation, filtering, and comparison blocks) of the second branch operate (slow reacquisition). The correct code phase is determined in correspondence of that correlation value greater than a threshold value. Particularly, the fast reacquisition is fast for stronger satellite signals, while the slow reacquisition is suitable for signals having lower strengths. In this case, the reacquisition, for that determined satellite, ends and the tracking step starts. When there are no acquisition values greater than the threshold value, the correlations calculation is repeated with reference signals having other code steps (or with an evolution about the prediction code).